The present invention relates to a brake fluid pressure controlling device that controls electromagnetic valves provided in a passage for brake fluid, thereby adjusting the brake fluid pressure in wheel cylinders.
A typical brake actuator mounted on a vehicle includes linear electromagnetic valves arranged in a passage connecting a master cylinder and wheel cylinders, and pumps that are located in the passage and connected to a part between the linear electromagnetic valves and the wheel cylinders. While the pump is operating, each linear electromagnetic valve is controlled to adjust the pressure difference between the brake fluid pressures on both sides of the linear electromagnetic valve, thereby adjusting the brake fluid pressures in the wheel cylinder.
These linear electromagnetic valves are activated in response to a command signal sent from an electronic control unit (ECU) for a brake actuator, for example, during the execution of the Adaptive Cruise Control (ACC). The faster the response speed of the linear electromagnetic valve, the quicker the brake fluid pressure in each wheel cylinder, or the braking force applied to the corresponding wheel, is adjusted to a target brake fluid pressure, or a target braking force. Japanese Laid-Open Patent Publication No. 1-145483 and Japanese Laid-Open Patent Publication No. 4-3824 disclose controlling methods for increasing the response speed of electromagnetic valves as typified by linear electromagnetic valves.
According to the prior art methods for controlling an electromagnetic valve, noise (for example, random noise that is an AC signal having random frequencies) is overlaid onto a command signal to generate an overlaid command signal, and the linear electromagnetic valve is controlled according to the overlaid command signal. The influence of the noise causes micro vibration of the valve body of the linear electromagnetic valve. Thus, when the valve body is displaced, it receives kinetic frictional force, which is smaller than static frictional force, instead of static frictional force. Compared to a case where the valve body receives static frictional force, the response speed of the linear electromagnetic valve is increased.
In addition to the urging force of the incorporated spring and electromagnetic force generated by the incorporated electromagnetic coil, the valve body of a linear electromagnetic valve receives the force of brake fluid discharged by a pump (also referred to as pump discharge force). The pump discharge force has a magnitude that varies in accordance with the operating state of the pump and acts on the valve body in a direction to increase the brake fluid pressure in the wheel cylinder. That is, if a linear electromagnetic valve is controlled according to an overlaid command signal, the force applied to the valve body is increased as the pump discharge force is increased. As a result, the micro vibration of the valve body is increased more than necessary, and the brake fluid pressure in each wheel cylinder, that is, the braking force applied to the wheel, may fluctuate.
To solve this problem, the amplitude of the noise may be set to a value that causes the brake fluid pressure in the wheel cylinder to fluctuate insignificantly even when the pump discharge pressure is great. However, if the linear electromagnetic valve is controlled according to an overlaid command signal when the pump discharge force is small, insufficient or no micro vibration of the valve body is generated. The response speed of the linear electromagnetic valve therefore cannot be increased.